Thinking about setting up a rooftop solar plant? You looked at your bills, your consumption pattern and decided that you need a 5-kw rooftop solar plant. You find an installer who surveys the roof and suggests only 3 KW, saying that a part of the roof has shadow in the morning. You look at the shadow analysis, and find that a small part of the panels will be shadowed. Wondering how much of an impact it can have?
Shadows on panels of a rooftop solar plant can happen due to three reasons –
1. Self-Shading: This happens when the distance between panels are too less shadow of one row of panels falls on another. Particularly during the winters when the shadows are longer, a lot of solar power plants face this issue. A better design, with carefully calculated distance between panels can avoid this.
2. Shadows of near objects: Every roof has many paraphernalia like trees, TV antennae, AC vent etc which can cast shadows on solar panels. To avoid these some of the sources of shadow (like AC vents) can be moved. But when the shadow is of an immovable object (like trees), the only way to avoid this is through better design
3. Shadows of far objects: At times, there can be a shadow on the roof because of nearby buildings, towers, light post etc. The best way to avoid this is simulating the shadow on the roof and avoiding the shaded areas. Hence, again, a better design is the best mitigation.
Thus, a well designed solar power plant would have no shadow on any of the panels.
Let us now understand the impact of shadows on generation. Following is the schematic diagram of a typical 5 kW rooftop solar plant with 16 panels of 330 Wp each.
Now, each panel in a string operates at roughly the same voltage (typically ~ 35 V). As long as a panel is generating (i.e. there is sunlight), this voltage remains roughly the same. The voltage during the noon when the sunlight is strong would be almost the same as that in the evening in the fading light.
The current of each string is the same since the current flowing through each of the panels. The value of the current depends on the radiation, i.e. the intensity of sunlight. Thus, during the noon when the sunlight is strong, the current would be higher, while in the evening in the fading light, the current would be lower. Typical current during the noon on a sunny day would be about 7 A.
- Total Voltage (V) = 16 panels X 35 volts per panel = 560 V
- Current (I) = 7 A
- Power generated by the plant (V X I)= 7 X 560 = 3920 Wp
Now what happens when there is a shadow on one of the panels? In the following diagram, one of the panels is in shadow, and can generate at say, 70% of the original current (i.e. 70% of 7 A = 4.9 A).
Now, in such a situation, the inverter decides to do one of the following:
Scenario 1: Reduce the current through the all the panels: The current through all the panels has to be the same. Hence, if one panel can generate at only 70% current, other panels, too, will operate at a reduced current.
We can understand this by taking the example of a pipe. If there is a blockage in the pipe, the water flowing through the pipe will get reduced.
A shadowed panel is exactly like the blockage in the pipe. Since due to the shadow, the current in the shadowed panel is reduced, current for all the other panels also have to reduce. Thus, in this case, the current through all the modules shall be only 70% of the original, i.e. 4.9 A. The power generated, too, shall come down by 30%, as the power reduces from 3920 Wp to only 2744 Wp.
- Total Voltage 16 X 35 = 560 V
- Reduced current = 4.9 A
- Power generated by the plant : 4.9 X 560 = 2744 Wp
- Loss in generation = 30%
Scenario 2: The shadowed panel is removed from operation: In such a case, the bypass diode of the panel is activated, and current does not flow through the panel.
In this case, the power generated reduced by 6.25% from 3920 Wp to 3675 Wp
- Total Voltage 15 X 35 = 525 V
- Reduced current = 7 A
- Power generated by the plant : 7 X 525 = 3675 Wp
- Loss in generation due to shadow: = 6.25%
One of the above possibilities would occur. Inverters from different companies have different ratings, and will lead to one of the above scenarios, albeit unpredictably. Hence, it is best to avoid shadows altogether.
From this example, we can see that a small shadow can result in a huge loss in power generation. The impact of shadows can be minimized by –
- Better design: We can ensure that no panel is placed in shadow. The shadows of every object can be accurately predicted through simulation software.
- DC optimizers or micro inverters: DC optimizers and micro inverters optimize the generation of each panel individually. Hence, in case of a shadow, the unshaded panels can generate normally. However, such solutions are very expensive.
The purpose behind setting up a solar plant is generating electricity from sunlight. The best solar plants are the ones where there is no shadow on any panels. A good design with where the following activities are done carefully ensures no shadow on any panel.
- Careful survey so that all the objects which can cause shadow are diligently considered
- Simulation of shadow of every such object
In summary, please do ensure that the solar plant has no shadows, at least during the major plant operating hours i.e., 9 AM to 3 PM. Do call us for any clarifications!